The present invention relates to an impurity doping apparatus utilized to form an impurity region having a desired conductivity type and resistivity in the fabrication of semiconductor devices such as bipolar transistors and Metal-Insulator-Semiconductor (MIS) field effect transistors.
The conventional impurity doping apparatus is typically composed of an ion implantation device. The ion implantation device is composed of an ion source, a mass spectrometer and an accelerating tube. The ion implantation is effected such that impurity atoms are ionized by the ion source, then an objective ion species is separated by the mass filter, and further the separated ion particles are accelerated by given energy with the accelerating tube to implant the ion particles into the semiconductor surface. The ion implantation device has advantages in that the doping amount of the impurity is controlled accurately, and the impurity doping can be easily effected through an insulating film. Therefore, the ion implantation device is generally used in conventional semiconductor fabrication processes.
However, conventional ion implantation devices have various drawbacks as follows. Namely, since impurity ions are accelerated to implant the impurity, the semiconductor surface of a sample may experience damage due to the kinetic energy of the impurity ions. Further, since the energy of the accelerated impurity ion particles fluctuates according to a Gaussian distribution having variance determined by the accelerating energy, the depth profile of the impurity atom density is rather broad in the semiconductor bulk, thereby making difficult the formation of a shallow impurity layer and the formation of a sharp depth profile in a deep part of the semiconductor layer. Moreover, since the ion particles are accelerated linearly, the introduction of impurity ion particles is unidirectional. Therefore, when carrying out oblique ion implantation in order to avoid channeling, asymmetricity is caused in the semiconductor device characteristics due to shadowing effects.